1. Field of the Invention
The present invention generally relates to a method for producing calcite-type calcium carbonate single crystal which can be advantageously used for various optical elements such as an optical polarizer, or the like. More particularly, the present invention relates to a method for producing a calcite-type calcium carbonate single crystal on an industrial scale by a typical hydrothermal synthesis method in which the aqueous solution of ammonium salt of mono-carboxylic acid is used as a solvent for the raw material to grow the single crystal on seed crystal under predetermined conditions such as the concentration and pH value of the solvent.
2. Description of the Prior Art
It is well known that a single crystal of calcium carbonate (CaCO.sub.3) commonly used as an optical element belongs to a calcite type structure according to a crystallographic view. Such calcite type crystals belong to a trigonal system and are in a thermodynamic stable phase of calcium carbonate under normal ambient temperature and pressure conditions. Since the calcite type crystals have inherently a high index of double refraction, their single crystals have been broadly applied to various optical devices.
As is understood by one skilled in this art, the above described phenomenon "double refraction" represents two different refractive beams which are generated to correspond to one incident beam through a single crystal. Particularly, the calcite type single crystals have been used as a polarizer prism for various optical devices owing to their high index of double refraction. As various devices of optics such as of laser optics, and of optical communication have been rapidly progressing and commonly being used in a widely enlarged market, materials for these optical elements having excellent optical properties have become to be required. Since the calcite type single crystals are known as an optically ideal material, it is easily anticipated that these materials will be greatly demanded within recent years.
As is well known, calcite is distributed as "limestone" in natural form throughout the world. Also in Japan, a great deal of "limestone" is produced. However, a relatively large size single crystal of calcite applicable to optical elements is rare and naturally produced in only Mexico and Brazil. Especially, the calcite type single crystal having a large size and a high quality tends to be less available.
Although many attempts to artificially grow the calcite single crystal having excellent optical properties have been conventionally carried out, none of these attempts have resulted in success. It is understood by skilled artisans that the conditions for useful artificially grown calcite-type calcium carbonate single crystal are to be possessed of properties such as the same index of double refraction as well as natural calcite; colorless and transparent to easily penetrate light in UV, visible light, and IR ranges; nonexistence of impurities, cracks, twin crystal and strains.
Calcite is decomposed at about 900.degree. C. under atmospheric pressure, so that it is impossible to directly form it from its melt. Conventional synthesis methods to produce calcite-type calcium carbonate single crystal have been known as the Gel Method (referring to H. J. Nickl and H. K. Henisch, J.Electroche.Soc. 116, 1258-1260, 1969), Flux Method (referring to J. F. Nester and J. B. Schroeder, Am.Mineralogist 52, 276-280, 1967), and Hydrothermal Method. However, Gel Method can not produce calcite-type calcium carbonate single crystal having a size large enough for optical elements. In the Flux Method, the resulted calcite single crystal tends to be mixed with impurities such as lithium which is caused by lithium carbonate used as the flux agent and thermally generated strain owing to a high temperature process. On the other hand, it has been known by one skilled in the art that the Hydrothermal Method is advantageous to produce a single crystal of a high degree of purity when the single crystal is not directly formed from its melted material by another method. Practically, an artificial single crystal of quartz has been industrially manufactured by this Hydrothermal Method.
The principle and process of this Hydrothermal Method will be briefly described.
First of all, a pressure container of an autoclave is filled with solvent. A seed crystal is disposed in the upper portion of the autoclave. A raw material for crystal growth is disposed in the lower portion of the autoclave. The temperature of the seed crystal portion is set slightly lower than that of the raw material portion when the temperature coefficient of the solubility of the raw material in the solvent is positive. The raw material at a higher temperature is dissolved into the solvent. The dissolved ions are moved upwards to the seed crystal portion by thermal convection owing to temperature gradient. The ion species arrive at the near area of the seed crystal portion and then are deposited on the surface of the seed crystal because the temperature of the seed crystal portion is lower than that of the raw material portion. Thus the seed crystal gradually grows up.
The solvent to be used can be selected from various inorganic aqueous solutions for increasing the solubility of the raw material. The growth rate of the crystal and the quality of the grown layer depend on the temperature of the seed crystal portion, the temperature difference between the seed crystal portion and the raw material portion, the type of solvent, and the concentration of the solvent.
Typical examples of the solvent used for growth of calcite-type single crystal by Hydrothermal Method are as follows. Aqueous solution of NaCl is disclosed in S. Hirano and K. Kikuta, Bull.Chem.Soc.Jpn. 1109-1112, 1987; and N. Yu. Ikornikova, Growth of Crystals, 3, 297-301, 1962. Aqueous solution of LiCl is disclosed in N. Yu. Ikornikova, Growth of Crystals, 3, 297-301, 1962. Aqueous solution of CaCl.sub.2 is disclosed in N. Yu. Ikornikova, Growth of Crystals, 3, 297-301, 1962. Aqueous solution of NaNO.sub.3 is disclosed in N. Yu. Ikornikova, Growth of Crystals, 3, 297-301, 1962. Aqueous solution of Ca(NO.sub.3).sub.2 is disclosed in S. Hirano and K. Kikuta, J.Cryst.Growth 94, 351-356, 1989; and S. Hirano, T. Yogo, K. Kikuta and Y. Yoneta, J.Ceram.Soc.Jpn. 101, 113-117, 1993. Aqueous solution of NH.sub.4 NO.sub.3 is disclosed in S. Hirano, T. Yogo, K. Kikuta and Y. Yoneta, J.Ceram.Soc.Jpn. 101, 113-117, 1993. Aqueous solution of K.sub.2 CO.sub.3 is disclosed in D. R. Kinloch, R. F. Belt and R. C. Puttbach, J.Cryst.Growth 24/25, 610-613, 1974. Aqueous solution of NH.sub.4 Cl is disclosed in N. Yu. Ikornikova, Growth of Crystals, 3, 297-301, 1962; and Brief of Lecture in 37th Artificial Crystal Debate, p53. Aqueous solution of carbonic acid is disclosed in M. Higuchi, A. Takeuchi and K. Kodaira, J.Cryst. Growth 92, 341-343, 1988.
Conventional methods for producing calcite-type calcium carbonate single crystal by the above described hydrothermal synthesis method are disclosed in, for example, Japanese Patent Laid-open Publication No. 61-215295/1986 entitled "Method of Growth in Chloride Aqueous Solution", Japanese Patent Laid-open Publication No. 62-113798/1987 entitled "Method of Growth in Nitrate Aqueous Solution", Japanese Patent Laid-open Publication No. 63-230593/1988 entitled "Method of Growth in Aqueous Solution of Calcium Nitrate", and Japanese Patent Laid-open Publication No. 64-28298/1989 entitled "Method of Growth in Aqueous Solution of Ammonium Nitrate".
The inventor of the present application has previously proposed a producing method of calcium carbonate single crystal by hydrothermal method using organic ammonium salt as solvent for growing the crystal, as Japanese Patent Laid-open Publication No. 6-316493/1994. According to this method, it was possible to grow calcite-type calcium carbonate single crystal with high growth rate under relatively low temperature and low pressure conditions in comparison with the conventional methods using inorganic salt aqueous solution as solvent. This resulted in grown layer with a high quality.
A naturally produced calcite-type single crystal, applicable for optical device should possess properties such as: colorlessness and transparence, nonexistence of: bubbles, cracks, twin crystal or strains, and relatively large, as optimum size. However, the single crystal satisfying these properties is rare and naturally produced in only the Republic of South Africa, Mexico and Brazil. Especially, the calcite type single crystal of large and high quality tends to be less than the industrial demand can satisfy.
In order to substitute for naturally produced calcite, many artisans unwillingly use the artificial calcite-type calcium carbonate single crystal produced by the hydrothermal synthesis method. But, as described above, conventional methods, using aqueous solution of inorganic salt as solvent, for growing calcite-type calcium carbonate single crystal are inferior with respect to the reason that the growth speed of the crystal is inherently slow, and when the concentration of the solvent and the temperature difference between the crystal growing portion and the raw material solving portion are increased to accelerate the crystal growth speed, the grown layer will contain the defects which are not useable for optical elements, or secondary crystals will occur. Further, if the temperature and pressure for crystal growth are increased to accelerate the crystal growth speed, a specially designed container withstanding high pressure and high temperature will be required. This will result in a rising cost to produce calcite-type calcium carbonate single crystal on an industrial scale. Consequently, the conventional method using inorganic salt type solvent is not available for the mass production of calcite-type calcium carbonate single crystal having such a high quality and large size for optical elements at a relatively low cost.